Spray coating apparatus

ABSTRACT

AN ELECTROSTATIC SPRAY GUN FOR FORMING A SPARY OF AIRATOMIZED, ELECTRICALLY CHARGED PAINT PARTICLES COMPRISES A GROUNDED METAL BARREL HAVING AT ITS FORWARD END A PASSAGE WITHIN WHICH THE SPRAY IS FORMED. PAINT IS SUPPLIED TO SUCH PASSAGE THROUGH A NARROW ANNULAR PAINT ORIFICE IN THE PASSAGE WALL AND IS ATOMIZED BY AN AIRSTREAM DIRECTED INWARDLY AND FORWARDLY OF THE PASSAGE FROM AN ANNULAR AIR ORIFICE LOCATED IMMEDIATELY REARWARD OF THE PAINT ORIFICE. AN IONIZING ELECTRODE SUPPORTED IN REAR OF THE PAINT AND AIR ORIFICES EXTENDS FORWARDLY ON THE AXIS OF THE PASSAGE PAST THE PAINT ORIFICE AND TERMINATES AT A POINT SPACED REARWARDLY FROM THE FORWARD END OF THE PASSAGE. AN APPROPRIATE VOLTAGE SOURCE MAINTAINS THE ELECTRODE AT ELEVATED POTENTIAL TO CREATE BETWEEN THE ELECTRODE AND THE WALL OF THE PASSAGE AN ELECTROSTATIC FIELD THROUGH WHICH THE SPRAY PASSES IMMEDIATELY AFTER IT IS FORMED.

United States Patent [72] Inventor DavidH.Badger [54] SPRAY COATINGAPPARATUS 12 Claims, 2 Drawing Figs.

52 user 239/15, 239/427.3,11s/121 511 1111. or B05b 5/02 50] FieldoiSearch :1: 239/3,15, 427.3; 118/121 [56] 1 References Cited UNlTEDSTATES PATENTS 3,169,882 2/1965 .luvinall et al 9/1967 Beach,Jr

3.339.841 239/3(X) FOREIGN PATENTS l,038,865 8/l966 Great Britain 239/15ABSTRACT: An electrostatic spray gun for forming a spray ofair-atomized, electrically charged paint particles comprises a groundedmetal barrel having at its forward end a passage within which the sprayis formed. Paint is supplied to such passage through a narrow annularpaint orifice in'the passage wall and is atomized by an airstreamdirected inwardly and forwardly of the passage from an annular airorifice located immediately rearward of the paint orifice, An ionizingelectrode supported in rear of the paint and air orifices extendsforwardly on the axis of the passage past the paint orifice andterminates at a point spaced rearwardly from the forward end of thepassage. An appropriate voltage source maintains the electrode atelevated potential to create between the electrode and the wall of thepassage an electrostatic field through which the spray passesimmediately after it is formed.

SPRAY COATING APPARATUS This application is a continuation of our priorapplication, Ser. No. 741,374, filed July 1, 1968, now abandoned.

BACKGROUND OF THE INVENTION While most electrostatic spray coatingsystems provide an electrostatic field extending to a grounded articlebeing coated and promoting the deposition of charged spray particles onthe article, it has heretofore been proposed to employ a spraying devicewhich would charge the spray before its discharge and to rely for theelectrostatic depositing effect solely on the electrostatic attractionof the grounded article for the charged spray particles. Such a devicecommonly embodies an atomizer adapted to discharge a spray of atomizedparticles through an electrostatic field maintained between an ionizingelectrode and a counterelectrode. The electrode and counterelectrode canbe located near the point of spray formation and hence can be ratherclosely spaced with theresult that a field of adequate strength tocharge the spray can be maintained at relatively low voltages. Further,it is possible to employ as the counterelectrode a grounded housingwhich encloses the charged ionizing electrode and, thereby protectingthe electrode against close approach to or by a grounded object orpersonnel, greatly lessens the possibility of dangerous sparks orshocks. One type of device employing an ionizing electrode enclosed in agrounded housing is shown in US. Pat. No. 2,302,289.

While prior devices of the type just mentioned possess the advantages ofrequiring only relatively low voltages and providing comparative safety,they possess a disadvantage, when used in spraypainting, in that paintcollecting on the counterelectrode under the influence of the field isthrown therefrom in the form of large drops which are objectionable bothbecause they mar the finish and because, if charged at all, they arecharged oppositely to the spray proper.

SUMMARY OF THE INVENTION In a preferred form of deviceembodying theinvention, the grounded barrel is formed of concentric, inner,intermediate, and outer metal sleeves. The inner and intermediatesleeves are constructed and arranged to define between them a passageand the annular orifice for the atomizing air, while the intermediateand outer sleeves define a paint passage and the annular paint orifice.Insulating means located in the inner sleeve rearwardly of the airorifice supports the ionizing electrode, which extends forwardly pastthe paint orifice and terminates within the barrel. The electrode ismaintained at high voltage by connection to the ungrounded terminal of ahighvoltage source in order to maintain between it and the groundedbarrel an electrostatic field through which the spray passes. Air ionsformed at the electrode and attracted to the grounded barrel charge thespray particles by ion bombardment with the result that when thedischarged spray is directed toward a grounded article to be coated thecharged particles will be electrostatically attracted to the article. Atits forward end the outer sleeve may be provided with diametricallyopposite orifices for the discharge of spray-shaping air, such orificescommunicating with one or more supply passages extending longitudinallyin the wall of the outer sleeve. If the gun is to be handheld, thebarrel may be provided with a grounded handle and with a trigger forcontrolling the discharge of paint and of atomizing and spray-shapingair.

DESCRIPTION OF THE DRAWING FIG. 1 is an illustration, largelydiagrammatic in character, illustratingan electrostatic paint-sprayingsystem embodying a handheld spray gun; and

FIG. 2 is a fragmental axial section, on an enlarged scale, showingdetails of the construction at the front end of the gun barrel.

DESCRIPTION OF THE PREFERRED EMBODIMENT The system shown in FIG. 1comprises a handheld spray gun designated in its entirety by thereference numeral 10 and including a barrel 11. and a handle 12.Conduits 13 and 14 extending from the lower end of the handle 12respectively connect the gun to a source 15 of air under pressure and asource 16 of liquid to be sprayed. A voltage lead 17 extends into thehandle 12 from one terminal of a voltage source 18, the other terminalof which is grounded as indicated at 19. A trigger 20- mounted on thegun controls the supply of air, liquid, and voltage in any convenientmanner. Exterior portions of the gun 10, including the barrel 11, handle12, and trigger 20 are of metal andare grounded as through a conductivesheave surrounding, but insulated from, the voltage lead 17. FIG. I alsoshows a grounded article 21 in position to receive spray from the gun.

The barrel ll of the gun, as shown in FIG. 2, comprises an inner sleeve22, an intermediate sleeve 23, and an outer sleeve 24, all of metal andconcentrically arranged. Opposed faces of the inner and intermediatesleeves 22 and 23 are spaced apart radially to define an annular,longitudinally extending air passage 25 terminating in an annular airorifice 26 communicating with the interior 27 of the inner sleeve. Theopposed surfaces of the intermediate and outer sleeves 23 and 24similarly define an annular, longitudinally extending paint passage 28terminating in an annular paint orifice 29 communicating with thepassage 27 immediately forwardly of the air orifice 26. The air passage25 and paint passage 28 communicate interiorlyof the gun with the airconduit 14 and the paint conduit 14 respectively. Paint supplied to thegun flows through the paint passage 28, is formed into an annular filmat the end of the intermediate sleeve 23, and emerges as the inner edgeof such film into the interior 27 through the paint orifice 29. As itthus emerges, it is impinged upon by the annular airstream dischargedthrough the air orifice 26. Such air orifice, as shown in the drawing,is formed by opposed frustoconical surfaces at the ends of the inner andintermediate sleeves to direct the annular airstream inwardly andforwardly of the gun, with the result that the paint particles formed byatomization at the paint orifice 29 are directed forwardly and towardthe barrel axis for emission from the barrel through an opening 30 atthe front end of the sleeve 24.

Mounted within the sleeve 22 well in rear of the air and paint orificesis a plug 32 of insulating material which supports a slim elongatedspray-charging electrode 33 connected interiorly of the gun to thehigh-voltage lead 17. The charging electrode 33 is of fine wire andextends forwardly to the vicinity of the paint orifice 29, but desirablynot far enough forwardly to receive any atomized paint.

If desired, the barrel 11 may include means for shaping the paint spraydischarged through the opening 30. Such means may, as shown in FIG. 2,take the form ofa metal ring 35 having a frustoconical, forwardlyflaring inner face 36. Diametrically disposed, oblique passages 37 inthe ring 35 open into the face 36 and communicate at their rear endswith an annular groove 38 formed in the front face of the outer sleeve24. The groove 38 in turn communicates with one or more air passages 39extending rearwardly through the wall of the outer sleeve 24 andcommunicating at their rear ends, through appropriate flow-regulatingmeans (not shown), with the air conduit 14. Air discharged from thepassage 37 converts the discharged spray from one circular cross sectionto one of flat or oval cross section, as is frequently desired inspray-painting operations.

The orientation of the flat spray just-referred to relative to thehandle 12 can be controlled by making the ring 35 angularly adjustableabout the axisof the barrel 11. To that end, it may be formed with anoutwardly projecting annular flange 41 secured against the front face ofthe outer sleeve 24 by a screw-threaded retaining ring 42.

With the electrode 33 connected to the voltage source 18, anelectrostatic field will exist between the electrode and the innersurfaces of the metal sleeves. As that field is concentrated at thepointlike tip of the electrode, a zone of high ion concentration iscreated on the axis of the barrel and for wardly of the paint orifice29. As the paint emerges from the orifice 29 it is impinged upon andatomized by the airstream emerging from the air orifice 26, and theatomized particles are propelled through the zone of high ionconcentration to be charged by ion bombardment. With the gun directed ata grounded article to be coated, the airstream discharged through theopening 30 will carry the atomized spray particles to the vicinity ofsuch article, and the charges on the particles will cause them to beattracted to and deposited thereon.

The radially inward component of velocity of the air emerging from theair orifice 26 initially propels the atomized paint particles toward theaxis of the passage 27 and opening 30, thus providing a dual benefit.The particles, while still in a state of relatively high concentration,are forced through the zone of high ion concentration at and adjacentthe centrally disposed tip of the electrode 33, thereby promotingeffective charging. In addition, the initial inward inclination of theairstream opposes the tendency of particles charged by ion bombardmentto deposit electrostatically on the inner surfaces of the groundedsleeves and ring 35, thus minimizing the deposit of particles onsurfaces at the forward end of the gun.

Since a substantial potential gradient exists at and immediatelyadjacent the paint orifice 29, the particles as originally formed maytend to possess an undesirable charge opposite to that which they lateracquire by ion bombardment. Accordingly, it is desirable to reduce asfar as possible the gradient of the field at and immediately adjacentthe paint orifice. This result can be attained by making the inwardlypresented surfaces of the barrel as nearly flush with each other as ispractical, thereby providing mutual shielding of the edges of thosesurfaces and reducing field strength and the undesirable'charging effectat the inner edge of the paint film emerging from the orifice 29.

Since the spray passage formed by the opening 30 and the forward portionof the interior 27 of sleeve 22 may have a relatively small diameter,say substantially less than 1 inch, an intense particle-charting fieldcan be produced between the electrode and the conductive walls ofdischarge passage 27 with only a few kilovolts applied to the electrode33. As the entire exterior of the gun is grounded, danger of undesirableelectrical discharges between it and other grounded objects is entirelyeliminated. By locating the tip of the electrode far enough rearwardlyfrom the front end of the gun, the danger that it might approach agrounded object closely enough to cause an undesirable discharge isrendered practically negligible. While it is desirable, for the reasonjust noted, to locate the electrode tip well back from the front end ofthe gun, a location too far back is to be avoided since the spray, afterits initial convergence, will expand; and undue expansion of the spraybefore its emergence from the gun would cause paint to be deposited inthe interior gun surfaces. Preferably, the axial distance between theelectrode tip and the front end of the gun should be at least equal tothe radius of the passage 27 and most desirably about equal to thediameter of such passage.

While the gun shown has been described, for convenience, as one for usein the application of paint, it will be understood that it may be usedwith other coating materials or, in general, for the creation of acharged spray of any liquid.

1 claim:

1. An electrostatic spray gun, comprising a barrel having a forwardlyopening discharge passage,

an annular liquid orifice in the wall of said passage, and

an annular air orifice in the wall of the passage adjacent to andrearwardly of said liquid orifice,

said air orifice being directed forwardly and inwardly of said passagewhereby air under pressure emerging from the air orifice will atomizeliquid emerging from the liquid orifice and propel the atomized liquidparticles forwardly and toward the center of said passage.

insulating means mounted in said barrel in the rear of said air orifice,and an ionizing electrode supported by said insulating means andextending in said passage to a tip which is located substantially on thepassage axis and forwardly of the liquid orifice.

2. A spray gun according to claim 1 wherein the discharge passage ofsaid barrelincluding said annular orifices to atomize liquid isconductive and comprises a terminus for an electrostatic field from theionizing electrode.

3. A spray gun according to claim 1 wherein the electrodetip is spacedrearwardly from the front end of the barrelby a distance substantiallyequal at least to the diameter o'f-said passage. 1

4. A spray gun according to claim 1 wherein the barrel in eludes, at itsfront end, an annular ring having a forwardly flaring, frustoconiealinner surface coaxial with said discharge passage, said ring beingadjustable about the barrel axis and being provided with passages forsprayshaping air opening in said inner surface and communicating with alongitudinal air passage in the barrel.

5. A spray gun according to claim I wherein surface portions of the wallof said passage adjacent said orifices are substantially flush with eachother.

6. A spray gun according to claim 1 wherein said barrel comprises inner,intermediate, and outer nested, coaxial sleeves, the opposed surfaces ofsaid inner and intermediate sleeves being spaced apart to define an airpassage terminating in said air orifice, and the opposed surfaces ofsaid intermediate and outer sleeves being spaced apart to define aliquid passage terminating in said liquid orifice.

7. A spray gun according to claim I wherein the electrodetip is spacedrearwardly from the front end of the barrel by a distance equal at leastto the radius of said passage.

8. An electrostatic spray gun comprising a barrel having a spraydischarge passage formed by a wall with at least a conductive forwardportion, and with a forwardly facing opening,

an ionizing electrode including a tip in said passage and rearwardly ofsaid opening and electrically insulated from said conductive portion atthe passage walls, said electrode and said conductive portion of thepassage wall being connected to a source of voltage to create a highlyionized zone in said passage,

an atomizer at the passage walland rearwardly of said tip and arrangedto form and direct a spray of coating material forwardly and inwardly ofsaid passage wall to pass through the highly ionized zone and outwardlythrough said opening.

9. An electrostatic spray gun, comprising a barrel having a forwardlyopening discharge passage,

a liquid orifice in said passage, and

an air orifice in said passage adjacent to said liquid orifice,

said air orifice being directed forwardly and inwardly of said passagewhereby air under pressure emerging from the air orifice atomizes liquidemerging from the liquid orifice and propels the atomized liquidparticles forwardly and toward the center of said passage,

insulating means in said barrel, and

an ionizing electrode supported by said insulating means and extendingin said passage to a tip, at least part of said discharge passage beingconductive and comprising a terminus for an electrostatic field from theionizing electrode.

10. A spray gun according to claim 9 wherein at least the tip of saidionizing electrode is located substantially on the axis of said passage.

11. A spray gun according to claim 10 wherein said barrel is grounded.

12. A spray gun according to claim 10 wherein said liquid orifice formsthe liquid into a thin film as the liquid emerges from said liquidorifice.

